Evidence for Internal Ion Pair Formation upon Insertion of Reactive Alkene in the Zirconium–Carbon Bond of the cp2Zr(μ-C4h6)b(c6f5)3 Metallocene-Boron-Betaine Ziegler Catalyst System

Treatment of the (butadiene)ML₂ complexes 1 ML₂ = Cp₂Zr ( a ), Cp₂Hf ( b ), and (.-C₅H₄CH₃)₂Zr ( c )] with B(C₆F₅)₃ gives the 1:1 addition products (CH₂CHCHCH₂–B(C₆F₅_₃)ML₂ ( 3a – c ). At –40°C the betaine complex 3a inserts one equivalent of methylenecyclopropane to give the regioisomeric insertion products 5a and 6a in a 60:40 ratio. These products exhibit the cyclopropylidene moiety in the α- and β-positions, respectively, relative to zirconium. The corresponding hafnocene complexes 5b and 6b are obtained in a 70:30 ratio starting from 3b . The reaction of 3 ( a – c ) with allene gives a single insertion product ( 7a – c ) in each case where the exo-methylene group is in the α-position to the metal center (2,1]-insertion). The complexes 5 – 7 are chiral. They all exhibit a pronounced ·-interaction of the internal –C⁴H=C⁵H⁻ double bond of the s̀-ligand chain with the metal center in addition to a metallocene/–C⁶H₂–B] ion pair interaction. The relative contributions of the cationic metallocene end of the dipolar complexes 5 – 7 are quite dependent on the steric and electronic properties of the respective metallocene units involved. This is revealed by a comparison to typical ¹³C-NMR parameters of the complexes 5 – 7 with a pair of suitable model complexes, namely the ethylene insertion product 4 into the betaine system 3a and its THF adduct 4 .THF.